Method for Correcting Downstream Deflection in Gas Turbine Nozzles

ABSTRACT

A gas turbine nozzle can be refurbished to reduce downstream deflection. The outer shroud of the gas turbine nozzle is held in a fixture, and then the nozzle is heated. The heated nozzle is then reshaped by a force exerted upon the inner shroud of the gas turbine, reducing the downstream deflection of the nozzle. After the deformation of the nozzle, an aft hook of the nozzle that has been adjusted by previous refurbishment efforts can be rebuilt to remove the previous adjustments.

TECHNICAL FIELD

The present invention relates to the field of gas turbines, and inparticular to a technique for refurbishing gas turbine nozzles.

BACKGROUND ART

In a gas turbine, gas is typically produced by the combustion of fuel.The gas is then passed over a collection of stationary nozzles, whichdischarge jets of gas against the blades of a turbine rotor, forcing therotor to rotate. The rotation of the rotor drives the external load ofthe turbine, such as an electrical generator.

One problem with gas turbines is that the gas loading on the nozzles andthe high temperatures in the turbine, eventually cause the stationaryturbine nozzles to deform. This is a particular problem with turbineswhere the nozzles are made of cobalt-based superalloys and use acantilevered design.

SUMMARY OF INVENTION

In one embodiment, a method of refurbishing a gas turbine nozzlecomprises mounting the gas turbine nozzle in a fixture, heating the gasturbine nozzle to a predetermined temperature range, and applying forceto the heated gas turbine nozzle distal from the fixture sufficient toreshape the gas turbine nozzle by a calculated amount.

In another embodiment, an apparatus for refurbishing a gas turbinenozzle comprises a mounting fixture, configured to hold an outer shroudof the nozzle, a hydraulic jack, positioned below an inner shroud of thenozzle, adapted to exert an upward force on the inner shroud, a heatsource, disposed with the nozzle, and a plurality of thermocouples,positioned with the nozzle and adapted for monitoring the temperature ofthe nozzle.

In another embodiment, an apparatus for refurbishing a gas turbinenozzle, comprises a means for holding a first portion of the nozzle, ameans for heating the nozzle, a means for exerting an upward force on asecond portion of the nozzle, distal from the first portion of thenozzle, and a means for supporting the second portion of the nozzle.

Other systems, methods, features, and advantages consistent with thepresent invention will become apparent to one with skill in the art uponexamination of the following figures and detailed description. It isintended that such additional systems, methods, features, and advantagesbe included within this description and be within the scope of theinvention.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an implementation of apparatusand methods consistent with the present invention and, together with thedetailed description, serve to explain advantages and principlesconsistent with the invention. In the drawings,

FIG. 1 is a drawing of an exemplary collection of stationary nozzles fora gas turbine, removed from the turbine for repair and refurbishment;

FIG. 2 is a radial view of a gas turbine nozzle illustrating thedeformation caused by downstream deflection;

FIG. 3 is a side view of a hook of a gas turbine nozzle, illustrating atypical conventional adjustment to the hook;

FIG. 4 is a radial view of a gas turbine nozzle illustrating theincomplete refurbishment of a conventional DSD refurbishment.

FIG. 5 is a side view illustrating a technique for reshaping a gasturbine nozzle according to various embodiments;

FIG. 6 is a side view illustrating an apparatus for reshaping a gasturbine nozzle according to one embodiment;

FIG. 7 is a side view illustrating an apparatus for reshaping a gasturbine nozzle according to another embodiment;

FIG. 8 is a side view illustrating the use of insulating blankets on thegas turbine nozzle according to one embodiment;

FIG. 9 is a side view illustrating one embodiment of induction heatingcoils for heating a gas turbine nozzle;

FIG. 10 is a top view illustrating a plurality of thermocouples used tomonitor the heating of the gas turbine nozzle according to oneembodiment; and

FIG. 11 is a side view illustrating a reshaped gas turbine nozzlemounted with a saddle fixture for inspection after reshaping accordingto one embodiment.

DESCRIPTION OF EMBODIMENTS

In a cantilevered gas turbine design, a ring 100, as illustrated in aradial view in FIG. 1, is composed of a plurality of circumferentiallyspaced apart stationary nozzles 110, each of which includes vanessupported between radially inner and outer bands or shrouds. Each nozzle110 is typically an arcuate segment with two or more vanes joinedbetween the inner and outer shrouds, as shown in more detail in FIG. 2.Each vane is an airfoil, and the vanes are typically cast with the innerand outer shrouds to form the nozzle.

Each nozzle 110 is cantilevered from the outer shroud, using hook tohold the nozzle in place. As the stationary nozzles 110 deform in adownstream direction, commonly referred to as downstream deflection(DSD), the nozzle 110 provides reduced axial clearances and radial sealclearances are compromised. As a consequence of the compromisedclearances caused by the DSD, sealing effectiveness is reduced, whichcan result in high wheel space temperatures.

FIG. 2 is an illustration of a radial view of a typical nozzle 110 inboth its original and an exemplary deformed condition caused by DSD. Thenozzle 110 is exemplary and illustrative only and other nozzleconfigurations are known in the art. Similarly, the deformation isexemplary and illustrative only, and each nozzle 110 may have adeformation that is different from any other nozzle 110. As shown inFIG. 2, the original configuration of the nozzle 110 as manufactured isshown in solid lines, and a deformed configuration is shown in dashedlines, with the deformation exaggerated for clarity of the drawing. Thenozzle 110 is fixed in place by the hooks 240 of the outer shroud 210when mounted in the turbine, and the vanes 230 and inner shroud 220 aredeflected downstream (to the right in FIG. 2). A box 250 engages withthe inner shroud 220, and contains a plurality of packing teeth 260.

Conventional refurbishment techniques attempt to rotate the nozzle 110into the original position by adjusting one of the hooks 240. Asillustrated in FIG. 3, a portion 310 of the hook 240 is machined away,and a pad 320 is built up by welding or brazing onto a radially inwardsurface 330 of the hook, causing the outer shroud 210 to rotate upwardlyfrom its original position when installed back in the turbine, whichbrings the deformed nozzle 110 back to the position illustrated bydashed lines in FIG. 4, which shows the nozzle 110 (in solid lines) inits original state and the refurbished nozzle 110 (in dashed lines). Asrepeated DSD refurbishments are performed using this conventionaltechnique, the repeated machining of the hooks 240 can also open segmentseal slots 340, as shown in FIG. 3.

But as can be easily seen in FIG. 4, the conventional adjustment of thehook 240 does not actually change the geometry of the nozzle 110, butmerely rotates the nozzle 110 to attempt to reduce the DSD. Furthermore,repeated conventional DSD refurbishment can change the outer sidewallflow path, and does not solve deformation problems such as the angledpacking teeth 260, which can contribute to high wheel spacetemperatures.

FIG. 5 is a line drawing in side view that illustrates one embodiment ofa technique for refurbishing the gas turbine nozzle 110, even one thathas been refurbished multiple times with conventional techniques. A worksurface 510, typically a workbench or table, provides a place to mountfixture 500 to hold the nozzle 110 above the work surface 510 asufficient working distance. The fixture 500 is composed of a plate 520,to which is attached brackets 530 and 540, configured to engage hooks240. The box 250 (not shown) is typically removed from the nozzle 110during the refurbishment. The fixture 500 is typically made of steel,although other suitably strong and heat-resistant materials can be used.The construction of the fixture 500 in FIG. 5 is exemplary andillustrative only and other configurations can be used. In particular,the fixture 500 can be of an integral construction or composed ofadditional elements than the elements shown in FIG. 5. The fixture 500can be welded or otherwise suitably attached to the work surface 510 asdesired.

Once the nozzle 110 is mounted on the fixture 500, the nozzle 110 isheated, then deformed in an upstream direction to counter the effect ofdownstream deflection, by force exerted from beneath the nozzle 110upwardly, shown by arrow 560. In some embodiments, an additional force,shown by arrow 550, is exerted onto the inner shroud 220 toward thefixture 510.

By pushing upward on the heated nozzle 110, the deformation caused byDSD is actually reversed, bringing the nozzle 110 closer to itsconfiguration when newly manufactured. Instead of merely rotating thedeformed nozzle 110, the nozzle 110 is reshaped to reduce or eliminatethe deformation, rotating the vanes 230 and inner shroud 220 relative tothe hooks 240 and outer shroud 210. After the nozzle 110 is reshaped, ifthe nozzle 110 had previously been refurbished by the conventional hookadjustment technique, the modified hook 240 is rebuilt by removing thepad 320 that was added to the undersurface 330 of the hook 240, andwelding back a pad onto an upper surface of the hook 240 where theprevious refurbishment had machined off a portion 310 of the originalhook 240. This rebuilding of the hook 240 can close segment seals 340that may have been opened by the earlier refurbishments.

The order of steps of the above technique of first heating the nozzle110, then reshaping it, and finally rebuilding the hook 240, can berearranged, by first rebuilding the hook 240, then reshaping the nozzle110 sufficiently on fixture 500 to rotate the vanes 230 and inner shroud220 back into their original position relative to the hook 240. But thereordered technique is not as good as the preferred technique, becausethe hook 240 cannot be positioned as precisely. When the hook is rebuiltlast, the desired position of the hook 240 can be calculated by anoperator of the reshaping apparatus, then the nozzle 110 reshaped toapproximately the right shape. After the heated nozzle 110 is reshaped,the hook 240 can be rebuilt to precisely the desired configuration,ensuring the nozzle 110, when put back into the gas turbine, is withinor close to the manufacturer's specifications.

Superalloys such as the cobalt-based superalloys frequently used in theconstruction of the nozzles 110 are not generally considered pliableunder heating, and are metallurgically created to attempt to avoiddeformation at high temperatures. So one of skill in the art would haveexpected that heating the nozzle 110 would not allow for the controlledforce reshaping necessary for refurbishment of the nozzle 110, but wouldhave caused fractures or other metallurgical damage to the nozzle 110.Applicants have tested the nozzle 110 and found no such damage to thenozzle 110 after the reshaping treatment.

In an embodiment where both the force 560 and force 550 are used, theinner shroud 220 can be caused to rotate in an additional dimension. Butin experimental testing, it was determined that use of the force 560 istypically sufficient, and that the rotation caused by the force 550tends to occur without the force 550 as the nozzle 110 is pushed closerinto its original configuration. In such an embodiment, illustrated inFIG. 7, a jack shaft 700 moves through a fixture 710, mounted to thework surface 510, under pressure from another hydraulic jack (notshown). Other techniques can be used to exert the force 550 on the innershroud 220.

Force 560 is applied by pressure from a hydraulic jack, typicallyraising one or more jack shafts 600 through the work surface 5 10.Preferably, at least two jack shafts 600 are used, exerting forceequally or differentially as desired on the inner shroud 220. With adifferential jacking, a desired radial rotation of the inner shroud 220and vanes 230 can be performed if needed. Once the nozzle 110 has beenjacked up sufficiently, jack stands can be inserted to allow the innershroud 210 to rest on the jack stands and withdrawal of the jack shafts600 while allowing the reshaped nozzle 110 to cool, before completingthe refurbishment by adjustment of the hook 240, as described above. Anyconvenient kind of jack stand can be used, for example, a screw-typejack stand, such as the jack stands 910 in FIG. 9 or a fixed heightstand, such as a cylinder machined to a predetermined height appropriatefor the nozzle 110. The jack stands can be affixed to the work surface510, or movable as desired.

To heat the nozzle 110 prior to reshaping, the nozzle 110 is firstinsulated using insulating blankets 800, as shown in FIG. 8. This is inpart for safety of the operator. Because the heating is done byinduction heating, the insulating blankets 800 can be applied beforewrapping the nozzle 110 with the induction heating wires, as shown inFIG. 8. The type and position of the insulating blankets 800 illustratedin FIG. 8 is exemplary and illustrative only, and any convenientinsulating blankets and positioning thereof can be used.

In one embodiment, the heating is achieved by using induction heatingcoils 810, which are typically composed of copper tubing, with a hightemperature insulation mesh surrounding the tubing. The tubing has highfrequency electricity provided to it, and cooling water on the inside,creating an electromagnetic effect that induces electrical currentswithin the part surrounded by the high frequency magnetic field. Becausean even temperature is desirable for the reshaping of the nozzle 110,the induction heating coils are wrapped around the nozzle 110. Variousconfigurations of the coils can be used, such as shown in FIGS. 8 and 9.In one embodiment, a configuration that is a pinched ovoid shape(roughly that of a peanut) is used, although other configurations can beused as convenient or desired. In some embodiments, such as shown inFIG. 9, the induction heating coils are wrapped around the nozzle 110 asa whole. In other embodiments, the induction heating coils can bewrapped through gaps between the vanes 230.

The use of induction heating is exemplary and illustrative only. Otherheating techniques can be used, such as quartz lamps, resistanceheating, flame heating, etc.

Typically, a plurality of thermocouples 1010, as illustrated in FIG. 10,allow an operator to monitor the temperatures at various locations onthe nozzle 110, to ensure no hot spots, and also to allow the operatorto control the power in the induction coils 810, bringing up the powerto the coils and the ramp rate to achieve the desired temperature of thenozzle 110. Any convenient thermocouple control and monitoring mechanismcan be used.

The temperature used for this technique is dependent upon the materialsused to construct the nozzle 110. For some nozzles 110, the superalloymetal is heated to approximately 2000° F., and generally between 1800°F. and 2100° F. The specific temperatures are exemplary and illustrativeonly, and different superalloy metals would require heating to adifferent range. For any nozzle 110, however, the nozzle 110 should beheated to a temperature above a hardening temperature, but below amelting point of the metal.

After refurbishment, the nozzle 110 can be checked for compliance withthe manufacturer's specifications by placing the nozzle 110 into atesting saddle such as the exemplary and illustrative saddle fixture1102 of FIG. 11, which mimics the surrounding components in the gasturbine. Then the positioning of the nozzle 110 can be checked atpredetermined points, such as points 1100-1195. For example, dimension1100 is one dimension that is directly affected by DSD. After reshaping,the nozzle 110 should be within manufacturer's tolerances.

While certain exemplary embodiments have been described in details andshown in the accompanying drawings, it is to be understood that suchembodiments are merely illustrative of and not devised without departingfrom the basic scope thereof, which is determined by the claims thatfollow.

1. A method of refurbishing a gas turbine nozzle, comprising: mounting the gas turbine nozzle in a fixture; heating the gas turbine nozzle to a predetermined temperature range; and applying force to the heated gas turbine nozzle distal from the fixture sufficient to reshape the gas turbine nozzle by a calculated amount.
 2. The method of claim 1, further comprising: restoring a forward hook of the gas turbine nozzle.
 3. The method of claim 2, wherein restoring a forward hook of the gas turbine nozzle comprises: machining off a first pad from a first portion of the forward hook; and welding a second pad onto a second portion of the forward hook.
 4. The method of claim 1, wherein applying pressure to the heated gas turbine nozzle sufficient to deform the gas turbine nozzle by a calculated amount comprises: determining a desired position of a first portion of the gas turbine nozzle relative to a forward hook of the gas turbine nozzle; calculating an amount of deformation of the gas turbine nozzle required to achieve the desired position; and applying pressure to a first portion of the heated gas turbine nozzle sufficient to move the first portion of the heated gas turbine nozzle to approximately the desired position.
 5. The method of claim 1, further comprising: holding the gas turbine nozzle in a reshaped position.
 6. The method of claim 1, wherein the predetermined temperature range includes 2000° F.
 7. The method of claim 1, wherein the predetermined temperature range is between 1800° F. and 2100° F.
 8. The method of claim 1, where heating the gas turbine nozzle to a predetermined temperature range comprises: insulating the gas turbine nozzle; positioning an induction heating coil with the insulated gas turbine nozzle; and electrically inducing heat in the gas turbine nozzle.
 9. The method of claim 8, where the induction heating coil is a pinched ovoid.
 10. The method of claim 1, wherein the gas turbine nozzle is within manufacturer's tolerances for the gas turbine nozzle after refurbishment.
 10. An apparatus for refurbishing a gas turbine nozzle, comprising: a mounting fixture, configured to hold an outer shroud of the nozzle; a hydraulic jack, positioned below an inner shroud of the nozzle, adapted to exert an upward force on the inner shroud; a heat source, disposed with the nozzle; and a plurality of thermocouples, positioned with the nozzle and adapted for monitoring the temperature of the nozzle.
 11. The apparatus of claim 10, further comprising: a holding unit, positionable with an inner shroud of the nozzle and sized to hold the inner shroud at a desired height above a working surface.
 12. The apparatus of claim 10, wherein the heat source is an induction heating coil.
 13. The apparatus of claim 12, wherein the induction heating coil forms a pinched ovoid.
 14. The apparatus of claim 10, wherein the gas turbine nozzle is within a manufacturer's tolerances after refurbishment.
 15. An apparatus for refurbishing a gas turbine nozzle, comprising: means for holding a first portion of the nozzle; means for heating the nozzle; means for exerting an upward force on a second portion of the nozzle, distal from the first portion of the nozzle; and means for supporting the second portion of the nozzle.
 16. The apparatus of claim 14, further comprising: means for insulating the nozzle.
 17. The apparatus of claim 14, wherein the gas turbine nozzle is within a manufacturer's tolerances after refurbishment.
 18. The apparatus of claim 14, wherein the means for heating the nozzle comprises a means for induction heating the nozzle. 